1. Field of the Invention
The present invention relates to an orthogonal transformation encoder for use in increasing a compression ratio in encoding a video signal with high efficiency
2. Description of the Prior Art
Since a video signal has a vast amount of data, a high-efficiency coding method is adopted for recording or transmitting the data. The high-efficiency coding method employs a technique of reducing the amount of data keeping the possible high quality of the image while avoiding visually conspicuous deterioration of the image quality. In line with the above technique, there is an orthogonal transform coding system as an essence of a bandwidth compression coding.
In the orthogonal transform coding system, a screen image having a prescribed size is divided into a plurality of screen image blocks and the data of each screen image block is subjected to an orthogonal transform coding thereby to obtain orthogonal transform coefficients. Then the obtained orthogonal transform coefficients are quantized in such a manner that the method of the quantization is a decisive factor concerning the deterioration in quality of the resulting image.
For example, according to "Adaptive Coding of Monochrome and Color Images" by Chen and Smith, IEEE Transaction on Communications Vol. COM-25, No. 11, November, 1997, pp. 1285-1292, energy of alternating current components is calculated from orthogonal transform coefficients thereby to control the bit amount for assignment based on the calculated result.
However, according to the conventional coding method mentioned above, although the energy in each block of the screen image is reduced, the block which includes important data of such as a ground or water surface image fatally deteriorates. Furthermore, the calculation of energy of alternating current components requires a calculation of sum of squares, and therefore the above conventional coding method is not assumed to be the best one in terms of circuit scale.